1. Field of the Invention This invention relates generally to a dual input clutch powershift transmission for a motor vehicle. In particular, the invention pertains to controlling the input clutches during gear ratio changes produced by such transmissions. 2. Description of the Prior Art A powershift transmission is a geared mechanism having two input clutches, which alternately connect a power source, such as an engine or electric motor, to two transmission input shafts. The transmission produces multiple gear ratios in forward drive and reverse drive though operation of gearing arranged in a dual layshaft configuration between the transmission input and its output. One input clutch transmits torque between theinput and a first layshaft associated principally with even-numbered gears; the other input clutch transmits torque between the transmission input and a second layshaft associated principally with odd-numbered gears. The transmission produces gear ratiochanges by alternately engaging a first input clutch and running in a current gear, disengaging the second input clutch, preparing a power path in the gearing for operation in the target gear, disengaging the first clutch, engaging the second clutch andpreparing another power path in the gearing for operation in the next gear. Because a dual clutch transmission has no torque converter to provide damping, such transmissions begin and end each gear shift with the holding clutch, i.e., the input clutch through which engine torque is transmitted to the transmission inputshaft for the target gear, slipping. To provide acceptable shift quality, it is necessary to maintain the correct slip without excess flare and without locking the holding clutch. At the same time, the input clutches must maintain sufficient torque atthe output shaft to provide consistent acceleration before, during and after the shift. The magnitude slip across the input clutches must be closely controlled of slip across the input clutches.